This invention relates to self-regulating heaters and more particularly to self-regulating heaters for interior well or exterior mount use for a refrigeration system compressor housing to maintain the lubricant therein above a predetermined temperature level.
In conventional refrigeration compressors, a refrigerant, such as one sold under the trademark "Freon" by E. I. du Pont de Nemours & Co., may, in liquid form, migrate from the condenser into the compressor lubricant. Then, when start-up of the compressor occurs, the sudden reduction in crankcase pressure may cause the refrigerant to boil, thus causing the lubricant to foam with consequent loss of lubrication to other mechanical parts of the compressor. It has been conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system which prevents the migration of refrigerant into the crankcase lubricant.
In the prior art, fixed constant-resistance heaters were used for heating the crankcase. However, these heaters were not self-regulating and thus required heat output temperature controls and the expense and low reliability associated with them. Self-regulating sump heaters such as disclosed in the co-assigned U.S. Pat. Nos. 3,564,199, 3,748,439, and 3,940,591 have proven useful in many applications. These self-regulating heaters employed a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity. Such heaters have a relatively low resistance at usual ambient temperatures, but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above a threshold or anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times the initial value.
Many prior art self-regulating heaters have had a thermal resistance of the device which is higher than desired. For example, these devices were made with a plastic housing and not filled with thermally conductive material because of the deleterious affect of epoxy on the PTC material. As a result a large PTC pill was needed to provide the heat required adding to the cost to manufacture and if attached externally also to the cost of operation. Additionally prior art devices used a different design depending on whether their use was internal or external.
Accordingly, it is an object of the present invention to provide an improved self-regulating heater.
It is another object to provide a self-regulating heater which can be used both in an internal well and as externally mounted for heating crankcase lubricant.
It is a further object to provide a self-regulating heater with a low thermal resistance.
It is still another object to provide a self-regulating heater which is easy to assemble, inexpensive in construction and reliable in operation. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
Briefly, the self-regulating heater of this invention comprises a D-shaped thermally conductive ceramic tube with a cap at one end defining a chamber therein. The chamber contains a high thermal conductivity ceramic based potting compound and a self-regulating heater member. The heating member comprises a plurality of PTC heating elements which are sandwiched between two heat sink plates of thermally and electrically conductive material. The plates preferably are of such a width to fit into the chamber and of such a length to extend past the respective ends of the elements to run nearly the entire length of the ceramic tube. One of the plates is positioned directly adjacent the internal flat portion of the D-shaped tube. A plurality of projections are provided on the plates to ensure good electrical connection between the elements and the plates when being secured together with a thermally conductive adhesive while also providing large enough gaps between the plates and the elements so that the adhesive can fill the gaps and avoid entrapped air. First and second terminal means are attached to the plates for supplying electrical power to the elements. After partial filling of the chamber with the ceramic potting compound and inserting the heating member with terminal means, the end of the tube opposite the cap is sealed with an RTV (Room Temperature Vulcanizing) silicone rubber compound to provide a water tight device.
The design of the heater provides for a low resistance device which can be mounted both internally and externally to a crankcase of compressor as well as easily and economically assembled. The D-shaped housing provides for easy insertion of the heating member and preferential location of one of the plates adjacent the flat wall housing. When using the heater externally the heater is attached with this flat portion of the D directly against the outside of the crankcase wall to provide maximum heat thereto. The low thermal resistance of the heater allows for the use of small heating elements which reduce manufacturing cost and in the case of the externally attached device also operating cost.